The present invention relates to an optical information recording medium for recording/reproducing optical information by applying a radiation beam to the medium, and to a method of manufacturing such an optical information recording medium.
The prior art optical information recording pits are disclosed in the U.S. Pat. Nos. 3,999,008, 4,238,803, 4,278,756 and 4,360,728 and the Japanese Patent Kokoku Sho. 56-23221.
An optical information recording medium stores a record signal (e.g., information signal and address signal) on one side of a transparent substrate made of glass or transparent hard plastics, in the form of pits (e.g., recess). The positions of pits are optically detected to reproduce the signal recorded on the medium.
Specifically, on the record surface of an optical information recording medium, there are formed a consecutive series of pits in the circumferential direction of the medium as shown in FIG. 8. Each pit has a front edge corresponding to a rise of a limiter waveform obtained by limiting a record signal, and a back edge corresponding to a fall of the limiter waveform. By applying a reproducing radiation beam to the series of pits, the front and back edges of pits can be detected based on a difference in intensity of lights reflected from a pit and reflected from the other area. Then, a limiter waveform can be obtained based on the detected front and back edges and hence a record signal can be reproduced. The above process will be described in more detail. As shown in FIG. 9a a reproducing radiation beam 40 applied to the area other than a pit is totally reflected by a reflective film (not shown) formed at the back of an optical information recording medium. All the reflected light fluxes are returned to the aperture of an objective lens 41. On the other hand, as shown in FIG. 9b, a reproducing radiation beam 40 applied to a pit 42 is diffracted by the pit 42 to produce diffracted light fluxes 43 having a large reflective angle. Most of the diffracted light fluxes 43 are directed outside the aperture of the objective lens 41 so that the light quantity of fluxes returned to the objective lens 41 decreases. In addition, the light amount is further decreased by the interference effect caused by the light fluxes returned to the aperture from the pit 42. Such a difference in light amount is detected by a detector such as a photodiode so that the presence/absence and length of the pit 42 can be detected to thus read the information stored in the optical information recording medium.
The amount of light reflected from a pit depends on the pit size. Although a detailed calculation is omitted, a difference in amount of lights reflected from a pit and from the other area becomes maximum the nearer the pit area becomes to that of the reproducing laser spot, assuming that the pit depth is .lambda.ml/4nl (ml is an integer) where nl is a refraction index of a transparent substrate and .lambda. is the wavelength of a reproducing laser beam.
Such an optical information recording medium, for example an optical disk is manufactured by the transfer technique using an original optical disk on which recesses substantially the same as those pits to be formed on the optical information disk have been formed. In a known method for cutting pits on an original optical disk, a radiation beam modulated by a recording signal is applied through a cutting head to a photoresist coating the surface of the disk in uniform thickness, while rotating the disk and moving the cutting head in the radial direction of the disk, to thereby obtain exposed pits corresponding to the pattern of the applied radiation beam.
Assuming that the light intensity distribution within a radiation spot is uniform, the light exposure amount to a photoresist is proportional to the product of a cutting radiation beam intensity and the beam application time. Therefore, assuming that a cutting radiation beam of constant intensity is applied to the photoresist layer at a constant linear velocity, the light exposure amount to the photoresist varies with time as shown in FIG. 10. Namely, the light exposure amount to the photoresist is small at the start point A where a radiation spot 43 starts to be applied because of a very short application time. Between the start point A and the following point B spaced apart from the point A by the spot diameter d, the light exposure amount to the photoresist becomes gradually large in proportion to the move distance l of the radiation spot 43 because the application time becomes correspondingly longer. Between the point B and the following point C, the exposure time of the photoresist is maintained constant because the beam spot application time is constant, i.e., the time being from when the front edge of the radiation spot 43 reaches a certain point to when the back edge passes that point. The exposure amount at the point C, spaced apart from the point D by the radiation spot 43 diameter d, is maximum, whereas between the points C and D, the light exposure amount to the photoresist gradually decreases in proportion to the move amount of the spot because the application time becomes correspondingly shorter.
In practice, the intensity distribution of a radiation spot 43 is not uniform, but it takes a Gaussian distribution where the central area has a highest intensity and the peripheral area has a gradually decreased intensity. Therefore, the light exposure amount to the photoresist of the radiation spot 43 at the start and end points A and D further decreases.
The size and depth of a pit after cutting through a photoresist is generally proportional to the radiation beam exposure amount to the photoresist. Therefore, with a smaller light exposure amount at the light application start and end points and a larger light exposure amount at the intermediate area, the sectional configuration of a pit 44 formed after the development of the photoresist has a sharp side face 44c as shown in FIG. 11 and gentle front and back end portions 44a and 44b as shown in FIG. 12. The plane configuration of the pit 44 has a wider intermediate portion and narrower front and back end portions 44a and 44b as shown in FIG. 13. The typical values of the lengths A, B and C for the configuration are 0.09 to 0.11 .mu.m, 0.35 to 0.38 .mu.m and 0.52 to 0.55 .mu.m, respectively.
An area where the pit after cutting has a constant depth is between the points B and C shown in FIG. 10, the area corresponding to the area from 44BE to 44CE shown in FIG. 12, and the shape thereof is shown in FIG. 13 at 44E. In case where the pit and the other area are distinguished by a detector such as a photodiode which detects a difference in amount of reflected lights, the detector is arranged to detect the depth difference between plane position 44a and depth position 44BE and the depth difference between plane position 44b and depth position 44CE, as shown in FIGS. 12 and 13. Consequently, the longer the distance from the 44a position to 44BE position or from the 44CE position to 44b position where the depth is not constant and gradually changes, the more the pit detection becomes incorrect. Thus, a bit detection error may occur and a C/N ratio for reproducing recorded information is degraded. The mean value of the depth D is 0.12 to 0.13 .mu.m and the angle .theta..sub.1 is 45 to 53 degrees.
Apart from the above, since the substrate of an original optical disk is rotated at a constant angular velocity relative to a cutting head, the relative speed between the photosensitive surface of the disk and a laser spot radiated from the cutting head becomes high in proportion to the distance from the rotation center of the substrate to the radiation center of a laser spot. In view of this, in a conventional cutting method, the light exposure intensity to a pit is changed in proportion to a distance from the rotation center of the disk to the radiation center of a laser spot, as shown in FIG. 15.
As previously described, the amount of reproducing radiation light reflected from a pit depends on the pit size. Although a detailed calculation is omitted, a difference in amount of lights reflected from a pit and from the other area becomes maximum when the pit depth is .lambda.ml/4nl (ml is an integer) and the pit depth W is about one third of the diameter 2w of the reproducing radiation spot, where nl is a refraction index of the disk and .lambda. is the wavelength of a reproducing laser beam. If a pit whose depth becomes gradually larger in the direction of movement a reproducing radiation beam as shown in FIG. 12 is read, the resultant signal waveform 45 becomes as shown in FIG. 14 which has gentle rising and falling edges as compared with an ideal pulse waveform 46. As a result, problems such as a large jitter of a readout signal and a poor C/N ratio arise.
If a high intensity cutting radiation beam is exposed to a photoresist for the formation of pits, a uniform pit width can be ensured from the front end portion 44a to back end portion 44b of the pit 44, and in addition a sharp configuration of the front and back end portions 44a and 44b can be ensured. Thus, the problems of gentle rising and falling edges of a readout signal can be eliminated. However, if such a high intensity cutting radiation beam is continuously exposed from the application start point A to the end point D, the pit width at the intermediate portion becomes large so that it becomes impossible to form a pit width most suitable for a signal modulation degree decided by a reproducing laser spot diameter. Further, this large pit width hinders the technical subject of a small pit size and an improved recording density.
Apart from the above, the pit length is now made shorter to improve the recording density. Therefore, of pits for a record signal having long and short wavelengths, a pit whose length is smaller than a reproducing laser spot diameter will probably be present. Particularly, the area in a reproducing radiation laser spot occupied by a pit having a shorter length than the laser spot diameter becomes smaller than that occupied by a pit having a larger length, respectively as shown in FIGS. 16a and 16b. In this case, a signal level reproduced from such a disk becomes small due to the lowered efficiency of interference and diffraction of the laser beam, resulting in difficulties in reading the information. This problem becomes more dominant as the pit shape becomes smaller as shown in FIG. 16c. The reason for this is that as shown in FIGS. 16a to 16c, the area at the pit bottom 44E becomes very small as compared with that at the upper pit plane 44, the ratio of the areas between 44a and 44BE and between 44b and 44CE where the pit depth is not constant but changes gently, to the entire area of the pit 44 becomes very large. Therefore, it is difficult in practice to use a pit 44 having a shorter length than that of a reproducing laser spot diameter.